1. Field
The disclosed concept pertains generally to electrical switching apparatus and, more particularly, to circuit breakers including a plurality of separable contacts.
2. Background Information
Circuit breakers have been used in alternating current (AC) applications and direct current (DC) applications. The applications for DC circuit breakers have been very small. With the larger use of alternative energy sources, such as photovoltaic applications, the DC applications are increasing. DC molded case circuit breakers have used mechanical thermal and magnetic trip units for overload and short circuit protection, while some DC air circuit breakers employ electronic trip units. Magnetic trip units instantaneously trip the circuit breaker when the current in the protected circuit exceeds a predetermined level. However, magnetic trip units are difficult to calibrate and are not as accurate as electronic trip units. Thermal trip units are less susceptible to nuisance tripping, but take a longer amount of time to trip the circuit breaker, and are susceptible to ambient thermal conditions causing accuracy problems. Because of these problems thermal and magnetic trip units are not typically used in the larger size and higher current rated circuit breakers in AC applications, but rather, AC electronic trip units, which use a current transformer to sense the AC current, are used.
Without a time varying magnetic field, the AC current transformer will produce no electromotive force with DC current, which makes the AC electronic trip unit inoperable in DC applications. Certain DC circuit breakers such as DC air circuit breakers have used a DC electronic trip unit in combination with a shunt to sense the DC current in the protected circuit. The DC electronic trip unit provides enhanced control and tripping accuracy of the circuit breaker over thermal and magnetic trip units. However, DC circuit breakers which include a DC electronic trip unit are costly as compared to the high volume and readily available AC electronic trip units.
Photovoltaic applications present difficulties for current DC circuit breakers. In photovoltaic applications, the short circuit current level can be relatively low (e.g., less than 200% of the rated current and usually about 125% to 135% of the rated current). Due to the relatively low short circuit current level, DC circuit breakers which use thermal and magnetic trip units are typically not desirable because it is difficult to set the magnetic trip unit precisely at these low levels and could cause excessive nuisance tripping and the thermal trip unit may not offer adequate protection due to the long time it takes to trip the circuit breaker. Additionally, thermal and magnetic trip units do not provide protection from arc faults. While a DC circuit breaker which uses a DC electronic trip unit can offer suitable circuit protection in photovoltaic applications, the cost of the DC circuit breaker with a DC electronic trip unit is a concern.
Self-powered electronic trip units derive power from the protected circuit in order to operate. When the current in the protected circuit drops too far below the rated current of the self-powered electronic trip unit, the self-powered electronic trip unit is no longer able to initiate a trip. An over current condition would not be present in the protected circuit if the current in the protected circuit was too far below the rated current of the self-powered electronic trip unit. However, an arc fault may still exist in the protected circuit even when the current is less than the rated current. It would be desirable to retain arc fault protection even as the current in the protected circuit drops.
There is room for improvement in electrical switching apparatus, such as circuit breakers.